media: Documentation: media: Fix v4l2-async kerneldoc syntax

Fix kerneldoc syntax in v4l2-async. The references were not produced
correctly.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>